Pressure sensors are employed in a wide range of pressure sensing applications in order to convert change in pressure of a fluid (e.g., gas, water etc) into a corresponding electric signal. Such pressure sensors typically include a diaphragm, a transmitter and a pressure detection unit to measure pressure of the fluid. The diaphragm includes a receiving surface for receiving the pressure with respect to the fluid and adding load to the transmitter. The diaphragm can be formed by etching a portion of wafer (e.g., silicon) with a desired thickness. The sensitivity of a piezoresistive based pressure sensor is highly dependent on the final diaphragm thickness and the position of a resistor relative to the edge of the diaphragm.
One prior art approach, for example, employs a KOH (potassium hydroxide) etching in order to form a pressure sensing diaphragm with an epitaxial etch stop on a wafer to control the diaphragm thickness. FIG. 1 illustrates a top view of a diaphragm pressure sensor 100 fabricated utilizing a prior art etching process. A wafer 115 (e.g., silicon) with an epitaxial layer 160 shown in FIG. 2 is conventionally etched in order to form a diaphragm 125 based on the KOH anisotropic etching method. The KOH etching, however, requires a relatively larger hole in the wafer due to 54.7 degree crystal etch plane. As a result, a chip for forming the diaphragm must be larger and therefore fewer die are produced per wafer, which crucially affects the device performance and manufacturing yield. In another prior art approach, a DRIE (deep reactive ion etching) process can be employed to produce a straight sidewall hole in the silicon wafer in order to create the diaphragm. One of the problems associated with such prior art approach is that the etch depth control is not enough to produce a satisfactory yield for high sensitivity devices.
Alternatively, a hybrid process combining the DRIE etching with KOH etching and the etch stop layer with an original configuration can be adapted for etching the diaphragm of the pressure sensor. FIG. 3 illustrates a top view of a diaphragm pressure sensor 200 fabricated utilizing a prior art etching orientation with the hybrid etching process. A wafer 215 (e.g., silicon) with an epitaxial layer 260 shown in FIG. 4 is conventionally etched in order to form a diaphragm 225 based on the DRIE etching process. The diaphragm 225 can be further etched utilizing the KOH etch finishing process in order to form uniform diaphragm thickness. The epitaxial layer controls the thickness of the diaphragm 225 and the edges of the diaphragm 235 are determined by DRIE etch size and the amount of time required to etch from the intermediate DRIE bottom and the etch stopping epitaxial layer. Eliminating the 54.7 degree hole in the back of the wafer 215 can reduce the die size. The hybrid process however, introduces a variable amount of 54.7 degree bevel 250 between the diaphragm edge 235 and the etch pit wall 230 dependent on the amount of over-etching required. FIG. 4 illustrates a side view of the diaphragm pressure sensor 200. The uncertainty of the position of a beveled edge 250 introduces uncertainty in the strain field. Such uncertain beveled portions 250 in major strain/stress areas of the diaphragm 225 may lead to inaccurate sensing of the pressure sensor 200.
Based on the foregoing, it is believed that a need exist for an improved method for etching a diaphragm pressure sensor in order to eliminate the uncertain beveled portion in a stress/strain area of the diaphragm, as described in greater detail herein.